1-Introduction
In the world economy, the study of playas is of great importance because many valuable minerals, including sulfate, sodium and potassium carbonate, gypsum, halite, nitrates, borates, as well as a number of rare and valuable metals such as lithium, rubidium, cesium and even uranium and some other rare metals are formed or accumulated in these environments. Iran has a large number of playas, brines, domes and salt pans (about 60 salt pans), and the importance of exploring and studying of the reserves and evaporates mineral potentials is not covered by anyone.2-Materials and methods
In this study, an ASD Field Spec3® Spectrophotometer was used in Laboratory of Graduate University of Advanced Technology to measure the spectra of samples. This spectrophotometer is capable to record spectra in the range of 350-2500 nm and contains 2151 bands in this range. The satellite data which was used in this study is ASTER images at 1T level of the summer season. These images were geometrically corrected and only atmospheric correction was performed on them, and the Internal Average Reflectance (IAR) method was applied for atmospheric correction. To understand the spectral behavior of evaporite minerals within the ASTER bands, these spectra were resampled based on the wavelengths of ASTER. Constrained Energy Minimization (CEM) method was used for mapping the evaporate minerals in six evaporate areas in Kerman province including Sirjan Playa, Khatoonabad Pan, evaporitic areas of Ravar, three of all Rayen craters, Koshkoieh Pan and Shahdad area. CEM algorithm highlights the known target signature by suppressing background signals and it constrains a specified target signature and minimizes other unknown signatures when the target spectra are provided as user end members. In order to validate the accuracy of the results of this method, the maps were compared with the ground data. In order to assess the mineralogy of the samples, after preparation, 36 samples from all samples taken for XRD analysis and were sent to the Mineral Processing Research Center of Iran. Based on XRD results all samples were categorized to four groups as Halite, Gypsum, Calcite and Thenardite.3-Results and discussion
Spectral investigations of samples that are not pure are complicated, but what is apparent in the samples of this study is the occurrences of strong adsorptions related to the presence of water, whether in the composition of minerals such as gypsum, bassanite or as adsorptions on minerals such as halite. Since evaporite minerals, especially halite, have a high tendency to adsorb water, this adsorption features were observed in almost all samples near wavelengths of 1400 and 1900 nm. Spectral curves of halite-dominant samples showed that in most of these samples absorption feature occurred within the band 6 of ASTER, but in the case of a pure halite sample, for example, sample ush5 which was taken from the center of Sirjan salt pan, and the sample Rn1, which was taken from crater No.1 of Rayen area, this absorption is occurred within the band 5 of ASTER. It can be deduced that by purifying the halite sample, the absorption feature position on the resampled electromagnetic spectrum tends towards the band with less wavelengths. Gypsum-dominated samples indicate that these samples have the absorption feature within band 6 of ASTER in the resampled spectra to the ASTER. The absorption of 2214 nm in sulfate minerals is due to the sulfate vibration.
CEM algorithm by maximizing the response of the known end members and suppressing the response of the composite unknown background, enhances the contrast between target spectra and the background. The ability of this technique has the advantage of being a straightforward technique that can be used for mapping with fewer field data (Gabr et al., 2010). Based on the results of XRD analyses of studied areas, four groups of endmembers including: halite, gypsum, calcite and thenardite were applied for this method. The maps showed that this method of classification has good results for detecting evaporate minerals.4- Conclusion
1- According to the results of XRD analysis in the studied evaporatic areas in Kerman province, the main evaporite minerals in each of the areas were identified. Halite is the most dominant evaporate mineral in Sirjan, and the minerals of gypsum, calcite and thenardite are respectively in the next level of frequency. In Khatoonabad, halite is not the main mineral and gypsum and calcite are the most abundant ones; besides, in Khatoonabad area, rare boron-bearing minerals such as inyoite, borcarite, aristarainite along with ulexite were observed. In Ravar, the most dominant evaporite minerals are gypsum and calcite, and halite in abundance is after these minerals. In this area, bassanite and boracite were observed that were not seen in the other areas.
2- The spectral features of evaporite samples of the study areas have been presented in Table 1.
3- In all studied evaporite minerals, absorption is observed in the range of about 1400 and 1900 nm. Absorptions in these wavelengths is due to the presence of water. In the investigated samples, even in minerals that do not have water in their structural composition, these two absorptions are also observed. In the case of minerals that do not have water in their composition, water is adsorbed by these minerals.
4- Evaporite minerals often show the absorption features in band 6 of ASTER, but the present study shows that this issue is different for halite. In the other hand, when the purity of halite sample increases, the absorption tends to be occurred within the band 5 of ASTER. In other words, by purifying the composition of the sample, the absorption band of resampled spectrum on ASTER tends to be in a lower wavelength.
5- Spectral features Studies of thenardite-dominant samples clarified that these samples exhibit a greater absorption width in the spectral range of water absorption rather than halite and gypsum.
6- Evaporite minerals have a high reflection. In the study of the reflection of the four major evaporite minerals of the study areas, it was found that halite and gypsum have the highest spectral reflection and thenardite has the least reflection.
7- According to the results of CEM method on ASTER images of evaporite areas of Kerman province, it was determined that this method is a suitable way for detection of the evaporite minerals.